Gas turbine engines produce power by expanding a hot combustion gas over multiple rows of rotating airfoils, often called blades, attached at their respective roots to a rotating shaft. Such blades are often cast from a superalloy material and may be coated with a ceramic thermal barrier coating material in order to survive the high temperature, highly corrosive combustion gas environment.
As the power levels of land-based electrical power generating gas turbine engines increase, the size of the rotating blades of such engines continues to increase, and the stresses imposed on the root attachment of the blades becomes a limiting design consideration. Additionally, longer blades are more prone to stall flutter under adverse aerodynamic conditions. It is known to include mid-span snubbers to provide a mechanical connection between adjacent blades in order to increase the stiffness of the blades, thereby making them more resistant to stall flutter. However, the additional weight of the snubber exacerbates the mechanical loads in the root attachment region.
The manufacturing of ever-longer gas turbine blades is also challenging the limits of known investment casting techniques. In particular, the ceramic cores used to define the internal cooling passages of cast gas turbine blades in the investment casting process are known to be relatively fragile and prone to damage during the wax and molten metal casting process steps. U.S. Pat. No. 5,505,250 discloses the use of platinum chaplets inserted into and extending from a surface of a ceramic core to provide point contact with a die surrounding the ceramic core during the molten metal injection step. The platinum chaplets dissolve in the molten metal, but they provide at least some support to the core during both the wax and metal injection steps, and they leave the outside cast surface of the metal smooth with no external penetration or void in the cast metal wall at the locations of the chaplets. However, the addition of the dissolved chaplet material into the molten cast metal may be undesirable for some alloys, and the innermost ends of the chaplets that are inserted into the ceramic core remain in the final cast product as an obstruction in the cooling passageway defined by the core.
Thus, improved designs and manufacturing techniques are needed in order to support the ongoing increase in size of gas turbine engine blades.